Quote Originally Posted by Velthice View Post
It basically boils down to, "I know what I'm doing."

For a little more in depth answer, you basically get the general idea of your tank's stamina in the first few pulls. Once you figure that out you pretty much know when you need to put out green numbers, especially in content that you're running for the nth time. Really tank hp dipping low may look perilous and reckless, but 99/100 it's super controlled and calculated with no one being in any real danger.
Pretty much this.

At least it's not like Pokemon where some abilities are based of how much HP you have, right? Thinking about it, now... that'd be pretty tricky to heal.